Has anyone swapped out their stock coil spring to the heavy duty tuftruck coil, the videos that I've watched states a 25-50% load capacity increase.

 

The springs may help with the trucks ability to carry a heavier load but they will not increase the factory stated payload of your truck. That is already determined by the factory and can't legally be changed.

 

Agreed. Your trucks load rating is not about springs alone. You will not safely increase the capacity by swapping springs. You will however have terrible ride quality when the truck is unloaded.

If you are looking to increase handling and ride quality when fully loaded but having it ride smooth unloaded, then buy a set of Air Lift air bags. I use them in my truck and they work beautifully. Install time is well under an hour with no tools at all. Cost is about $80.00 and they are fully adjustable. I haul 1,200 lbs of steel around fairly often and the truck sits level and rides normally.

 

I went with the skyjacker +4 a few years ago; it squatted pretty bad with my HD in the bed, never looked back or squatted again.